But what does it mean for a beer to be barrel-fermented? And how is that different from barrel-aging?

Let’s first remind ourselves what fermentation is at its most basic: The process by which yeast eat sugars found in wort and convert them into alcohol. There are two major places where fermentation can occur during the brewing process: in stainless steel tanks (the most common) or in wood vessels such as barrels, foeders or puncheons. Brewers reserve the latter technique almost exclusively for sour and wild ales—that is, beers brewed with wild yeast and bacteria other than saccharomyces, the “clean” brewer’s yeast. Most “clean” beers ferment in stainless steel tanks, and could then be transferred to barrels for aging (if the brewer chooses) after primary fermentation is over and the yeast have nearly completed their work. Of course, for every rule there is an exception. Seattle’s Holy Mountain Brewing fermented a California Common in third-use bourbon barrels; since then, they’ve moved on to ferment a wide range of “clean” beers in oak. Recently, they’ve had success using a cognac barrel to primary ferment hoppy beers with house ale yeast strains.

“The differences are subtle, especially with some of the lagers, pale ales and IPAs that we’ve done in oak. We’re looking for just a slight oak presence, if any,” says Holy Mountain co-founder and head brewer Colin Lenfesty. “For beers that are fermented in one of our oak cognac tanks, we’re still getting a hint of spirit character. It’s very subtle, but it integrates well and creates a layer of complexity that we wouldn’t get if the beer was fermented in stainless.”

Clearly, if you ask 20 brewers how exactly they make their beers, you’ll get nearly 20 different answers. Brewers of sour or wild beers especially can take many routes when it comes to contact between beer and barrels. Option one: They can add wort and a dose of yeast and bacteria straight to the barrel to let it undergo primary fermentation there. This beer would probably be described as barrel-fermented. Option two: They can let a beer undergo primary fermentation in stainless steel and then add it to a barrel along with a dose of yeast and bacteria (and possibly fruit or other additions) for secondary fermentation and aging. This beer would probably be described as refermented in the barrel or secondarily fermented or aged on fruit in the barrel. Option three: They can add wort to a barrel or other wood vessel and allow naturally present bacteria and yeast to ferment it, a la Belgium’s lambic producers. This beer would probably be described as a lambic-style or spontaneously fermented beer, and it’s not terribly common in the States. When most breweries refer to oak- or barrel-fermented beer, they’re referring to beer primarily fermented in the wooden vessel (option one). That’s the definition we’ll stick with here as well.

When most brewers refer to barrel-aging, especially in terms of stouts, porters and other clean base beers, they’re talking about beers that have already finished fermentation and are fully attenuated. At Denver’s Banded Oak Brewing Co., owner Will Curtin says in terms of most straight barrel-aged beers, he waits for yeast to flocculate (or clump together) in the stainless tanks once fermentation is complete; he’s then able to draw that yeast off and rack “mostly clear beer” into his barrels. At this stage, before the beers enter the barrels, you could drink them as-is; the barrel is there to add flavor to a beer or to smooth out the alcohol in a high-octane beer, not to act as a fermentation vessel.

“If you have really fresh barrels, I consider that barrel the fifth ingredient after water, hops, malt and yeast,” says Andy Parker, chief barrel herder at Avery Brewing Co. “A vast majority of the time, we build a recipe from the ground up around those barrels. When you have the potential to make a beer that you’ve never tasted anything like it, that’s where that really fun part comes in and why barrel-aging is so much to play with.”

Both barrel-aging and fermenting in barrels have their own risks and rewards. Barrel-aging risks oxidizing the beer, overpowering its base nuances or exposing it to undesirable microbes. Fermenting in barrels can be tricky because wood is less airtight and more prone to environmental fluctuations than steel. But the benefits obviously outweigh the risks for breweries that have built their entire lineups around barrel fermentation.

“We chose this method because I really like funky saisons, and any beer I’ve had that I knew was fermented in wood I thought had a cool extra layer of complexity to it,” says Kevin Osborne, co-founder and brewer at L.A.’s Cellador Ales, which is dedicated entirely to barrel-fermented beers. “I think you get a lot more variety between our barrels than if we had started with a primary fermentation [in stainless] and then split it into barrels. If we use the same recipe and same yeasts, every barrel has uniqueness to it and does its own thing. It’s cool for blending, but also more scary in a lot of ways. Everything’s a lot less predictable.”

For sour and wild ales, variation between barrels can be an asset because it offers many components for a final blend. Inherent in that variation, though, is less control over fermentation than a brewer would have using stainless steel. Temperatures in barrel warehouses fluctuate and wood isn’t impermeable like steel fermentation vessels, which are airtight and can be held at a constant temperature.

“We have an AC unit and some gas heaters for our space, so I’m able to keep it between 70-80 degrees most of the time, but that varies,” Osborne says. “In the summer, it’s probably closer to 80. Beers that ferment closer to 70 are going to give off a different character than the ones that ferment in the summer. There’s a story for each one in terms of why it tastes the way it does.”

Most brewers who’ve used both steel and wood for primary fermentation cite major differences between the vessels outside those related to temperature control. At Cellar West Artisan Ales in Boulder, founder Zach Nichols focuses entirely on barrel fermentation. But the brewery does also have a two-barrel stainless brewing system that it uses for lab experiments, and Nichols says he’s fermented the same wort and yeast/bacteria mix in both stainless and barrels to very different results.

“With oak fermentation, really what I notice is you’re opening Pandora’s box a bit to some more wild inoculations. Oak barrels are porous and microbes can pass in and out somewhat freely from the oak, in a way you can’t with steel,” Nichols says. “That can be detrimental if you get something you don’t want in there, or it can be beneficial if some microflora from your area adds some terroir to the beer.”

Some brewers say barrels “breathe,” in a way. They’re not 100 percent airtight, so very small amounts of oxygen flow through the wood in a process often referred to as micro-oxidation. This is especially important for wild ale brewers, most of whom include Brettanomyces yeast in their beers.

“Brett does need oxygen over time. When you’re letting it sit in that oak barrel, it does breathe with temperature fluctuations and keeps that Brett healthy and chewing away and adding unique character to beer,” Nichols says.

Nichols also hopes some of his house culture, a mix of wild yeast and bacteria, seeps into the barrels’ wood to create a “history” linking all of his beers together.

“We use barrels several times over for fermentation, so we have this stock of bugs that begin to live in the oak,” he says. “That’s one of the main reasons that historically Belgian brewers have used oak barrels for fermentation in lambic production. You have this chain of history from the bugs living in the oak, keeping that continuity.”

This idea—that yeast and bacteria take up residence in the wood and mix with other microbes present from the barrel’s former life as a wine cask—isn’t shared by all wild ale brewers. Some assiduously clean their barrels in between uses to minimize the potential for undesirable bug buildup.

It’s just one of the many choices that brewers must make in developing their own methods for fermenting in barrels. As with any beer that touches wood, these processes vary from brewer to brewer and beer to beer. Next time you read a beer label or description that mentions barrels, ask the brewer exactly how he or she is using them. You’ll learn a lot, and you’ll realize just how much more there is to explore in terms of these complex processes.

Three to try

Jester King SPON – Methode Gueuze 2016 (spontaneously fermented, blended wild ale)
Though crafted in Austin, this beer’s funk-for-days nose is pure Belgium: Lemon juice and wet hay overlay split oak while sweet verbena, pear skin and peach syrup meet the underside of a horse saddle. Even more lemon emerges on the palate (though there’s less acidic burn than one would imagine), then rhubarb and dry vanilla bean fade as oaky tannins bite. It’s an outrageously vibrant and accurate version of this hard-to-replicate style.

Cellar West God’s Eye 2017 (oak-fermented wild ale/porter with blackberries)
A purplish-brown hue clues you into the fruit addition, as does the nose: smooth milk chocolate first, followed by sweet blackberry jam, toast char and sarsaparilla with soft peanut scents around the edges. On the sip, wood character emerges right away as a sort of fresh oak greenness before blackberry skin quickly takes the reigns. The swallow is where things get interesting, as the dark malts unfurl with notes of cocoa and the lightest hint of black licorice. Tartness is very mild, more tannic than acidic. Though sweet blackberry is perceptible up front, the finish is very dry, with a focus on malty char evincing a well-made porter base.

Triple Digit Belsnickel 2016 (brandy- and bourbon- barrel-aged strong ale)
Time spent in brandy and bourbon barrels gives this strong ale a fruitcake aroma that melds scents of fig, orange and rum-raisin. The flavor’s energized by impish spice additions that layer ginger, nutmeg and orange peel between milk chocolate, sugary vanilla, fresh coconut and robust red wine. Boozy but not hot, this is one for sipping in front of a fire.

In my ongoing quest to visit breweries all across this great land, I have now surpassed the 400 mark, and they’ve been spread across 37 states and 175+ cities. To celebrate this landmark, I’ve put together a ‘Special Edition’ of Brewery Travels: A rundown of my favorites in each of the states visited so far.

I’ve worked in craft beer for nearly five years now. I’ve had the fortune to try some truly amazing brews: Pliny the Elder, Heady Topper, Bourbon Barrel Aged Expedition Stout. Supplication? I’ve got one in my mini-fridge. The reason I’m telling you this is because I want to frame my statements here properly. I’ve had good beer, trust me. The best beer I’ve ever had, though, was a Miller Lite.